This invention relates to the production of water soluble, storage stable resins.
The three most essential features of a water-soluble resin system are water-solubility, satisfactory end-use properties, and storage stability.
Water-thinnability can be achieved in various ways;
1. The formation of polymer or polyester chains carrying free acid groups along the length of the chains; solubility in water being achieved by the addition of amine bases such as ammonia, triethylamine. The acid groups in question are usually carboxyl groups originating from di- or polycarboxylic acids incorporated in the resin formulation, but can also be sulphate groups added onto the polymer or polyester molecule, usually at points of unsaturation, (so-called anionic systems);
2. POLYMER OR POLYESTER CHAINS CARRYING HYDROXYL OR POLYETHYLENE OXIDE GROUPS, AND THUS GIVING WATER-SOLUBILITY DUE TO THE AFFINITY TO WATER OF THE HYDROXYL OR POLYETHOXY GROUPS (SO-CALLED NONIONIC SYSTEMS);
3. Chains carrying amine groups; solubility in water being obtained by quaternisation with acids (so-called cationic systems);
4. DISPERSION IN STABLE AQUEOUS EMULSION; HERE INSOLUBLE POLYMER OR POLYESTER MATERIAL IS HELD IN SUSPENSION IN FINE PARTICLES IN WATER WITH THE AID OF SURFACTANTS AND VISCOSITY CONTROLLING PROTECTIVE COLLOIDS.
Various resin systems can be designed incorporating the above means of achieving water-thinability. Well known examples include acrylic- or vinyl copolymers with unsaturated acids, plasticized phenolic mixtures, maleinised and fumarated oils, alkyds based on tri- or polybasic acids, alkyds incorporating polyethylene or polypropylene glycol or monoether derivatives, alkanolamine condensates and emulsions or lactices of alkyds and of copolymer materials such as P.V.A., polystyrene, polyacrylics.
In the case of resins for paints, film-formation presents special problems with water-soluble systems by virtue of working with a system which must be ideally perfectly water-thinnable for application, but finally after drying or curing must be as resistant to water as any conventional solvent based system, and must have equivalent mechanical properties (such as adhesion, impact resistance, hardness, flexibility etc.). It is evident for this reason that latex systems are favored for air-drying coatings; such products can be made with suitably low quantities of surfactant and protective colloid so as to give a water resistant film after coalescence of the fine polymer particles on drying. In this field little success has been achieved with products made water-soluble by the means listed under 1, 2, and 3 above, because such products to not give satisfactory weather resistance. The main interest in considering such products for air drying paints is for gloss and brushability which cannot be fully obtained even by the newer modified latex systems.
In the case of stoving systems the amine base solubilized resins are more interesting, and in fact offer the widest scope for development of all the means available for obtaining water-solubility. The large number of acid groups present help to catalyze the reaction between the hydroxyl groups which are usually also present in large numbers and co-reacting amino-resins. In particular in the case of carboxyl acid groups there is the further possibility of the acid groups reacting during curing with the hydroxyl groups to give an alkali resistant film. If such resins are readily water-soluble with reasonably low amounts of volatile amine bases (B.P. less than 150.degree. C), curing is complete after a conventional stoving cycle. There remains the problem of formulating to high acid number a resin giving the required mechanical properties with reasonably low proportions of amino-resin, and having acceptable storage stability.